Mathematics behind creating shapes using math nodes
If you want to make simple shapes here is a recommendation - https://www.youtube.com/watch?v=BqijDcTdfZ8 but i can explain here , if you separate the xyz of texture coordinate using  a Separate xyz node and then connecting it to a compare node(math) , you can get a black strip on a black plane , this could be used to make a square by subtracting another such white stripe from it , i really recommend watching the given tutorial. IN the future , you might want to make your own shapes  and as you have mentioned in the link , the topic used in the equation is Linear and quadratic equations you can by the way trace equation in node , like say you have a value node , assume it to be x then adding math node to divide it by 7 , then square the result , it can be done , but if you want to verify the equation , proper linear and quadratic equation knowledge is needed to interpret them